The present invention relates to a mounting assembly for use with resiliently mounted blades located opposite to a forming shoe in the forming section of a twin fabric papermaking machine. In such a machine, since the blades are located on each side of the two fabrics, each blade contacts only the machine side of the nearer fabric. This invention is particularly concerned with a flexible mounting which allows the blades mounted thereon to conform locally to Z-direction variations in the path of the forming fabrics across the width of the moving forming fabrics. The flexible mounting allows the blade to move so that the angle of wrap of the forming fabrics as they pass in sliding contact over the fabric contacting surface of the blade is maximized at one of the blade edges and minimized at the other as the blade is displaced further towards the fabrics.
Many structures have been proposed for mounting static forming fabric supporting elements, such as blades, which are used in the forming section of a two, or twin, fabric paper making machine, in which the aqueous stock is injected or conveyed into the space between two opposed forming fabrics. In the forming section, blades in contact with the machine side of each forming fabric are used to improve formation and to assist in the removal of fluid so that an incipient paper web is formed. In certain forming sections of this type, the two forming fabrics moving as a pair are caused to wrap about blades located on both sides of the two forming fabrics and thus follow a somewhat zigzag path through the forming section.
In twin fabric forming sections equipped with opposed blades, a first set of flexibly mounted blades is located on one side of the fabrics, and a second set of more or less rigidly mounted blades is located on the other side of the fabrics. Suitable machine structures are provided to support both sets of blades, which are collectively referred to as forming shoes. The flexible mountings often utilize arrangements of pressurized hoses or springs to urge the blades into contact with the adjacent one of the two forming fabrics. This invention is concerned with an improved means for flexibly mounting these blades, in which the mounting is flexible in essentially two ways. First, the mounting means of this invention permits the blade to be forced into uniform intimate contact with the machine side of the nearer forming fabric across the width of the forming section. Second, the mounting allows parts of the blade to deflect in the Z-direction locally across the width of the forming fabric in response to localized variations that may occur, and still maintain the blade in intimate contact with the face of the forming fabric.
In the context of this invention, the following terms have the meanings given:
xe2x80x9cmachine directionxe2x80x9d means a direction substantially parallel to, or coincident with, the overall direction of travel of the pair of forming fabrics through the forming section;
xe2x80x9ccross machine directionxe2x80x9d means a direction essentially within the plane of the forming fabrics and perpendicular to-the machine direction;
xe2x80x9cZ-directionxe2x80x9d means a direction essentially perpendicular to both the machine and cross machine directions, and
xe2x80x9cbladexe2x80x9d means any stationary fabric contact element used in the forming section of a twin fabric paper making machine.
Many arrangements have been proposed in the prior art for resiliently mounting the fabric contacting elements used in a twin fabric forming section. These known mountings locate the blades in the cross machine direction, and provide for movement of the blades in more or less the xe2x80x9cxe2x80x9cZ-directionxe2x80x9d. However, in these mountings, Z-direction movement of the blades often involves frictional sliding between fixed and moveable parts of the mounting, which is frequently hampered by clogging of the mechanism by fibers and other matter. Further, in the known prior art arrangements the sliding movement components are generally stiff and inflexible, and thus do not allow for any localized flexing of the blade in the Z-direction in response to localized changes in conditions. Consequently, when localized misalignment of the blade with the forming fabric occurs, areas of poor formation and uneven drainage occur across the incipient paper web.
Many of the prior art mounting means cause the blade to move towards the fabrics in the Z-direction, and maintain the fabric contacting surface of the blade generally perpendicular to that direction. Further Z-direction movement causes the fabrics to wrap over the blades more or less symmetrically, which increases frictional contact between both the leading and trailing edges of the blade and the machine side of the forming fabric. This contact is known to accelerate the rate of fabric wear. To reduce the rate of fabric abrasive wear due to the wrap angle at the sharp leading edge, the radius of curvature of the blade leading edge is often increased. This has been found to create new problems, because any fluid that was adhering to the machine side surface of the forming fabric will be propelled back into the stock by the rounder edge as the fabrics pass over the fabric contacting surface of the blade. This phenomenon has been found to impair paper formation.
Kade et al, in U.S. Pat. No. 4,865,692, disclose a support structure for a blade, for use in a conventional single fabric open surface forming section. In this structure, two C-section beams extending across the width of the machine are interlocked to provide an essentially S-shaped structure. The upper C-beam carries the blade, and the lower C-beam is mounted onto the drainage box. The two C-beams are joined together by a flexible spring steel strip, and are urged apart by a clamping element so that the two C-beams engage to form the S-shape. The clamping element is typically an inflatable hose. An adjusting beam is also located between the two C-beams, which also carry stop surfaces. By moving the adjusting beam, different stop surfaces are engaged by pressurizing the clamping means, thus altering the angle of inclination of the fabric contacting surface relative to the machine direction of movement of the fabric. However, this structure only allows a small angular change, the axis about which the angle changes is not defined with any precision, and interlocking of the two C-beams into an S-shaped structure precludes any movement in the Z-direction. Further, although movement of the upper C-beam does not involve any sliding contact, that movement is controlled by the sliding adjusting beam.
The present invention seeks to provide a mounting for a blade in the forming section of a two fabric paper making machine which allows for movement of the complete blade in the Z-direction, localized flexibility of the blade across the width of the forming section in the Z-direction, and some freedom to alter the angle of inclination of the contacting surface in relation to the undeflected path of the forming fabrics. The mounting allows the fabric contacting surface of the blade to be urged into contact with one of the pair of forming fabrics so that it is initially oriented substantially parallel to the contacted surface of the undeflected forming fabric. The blade mounting provides the option of allowing the blade to engage the fabrics so that their angle of wrap about the blade is minimized at the leading edge, and maximized at the trailing edge. This is because the mounting allows the blade to rotate over an arc as it is displaced towards the fabrics. This arrangement reduces the angle of wrap of the forming fabrics about the leading edge of the blade, and permits the use of a relatively sharp doctoring leading edge on the blade. Alternatively, the mounting allows the blade to be engaged with the fabrics so that the wrap angle is maximized at the leading edge, and minimized at the trailing edge. If this arrangement is used, then it is recommended that the radius of curvature of the doctoring leading edge of the blade be increased to reduce fabric wear. The mounting additionally allows the blade to flex locally in the Z-direction, in response to local changes across the width of the forming section, so that the blade conforms more reliably to the line of fabric travel. Since the mounting involves no parts moving in sliding contact, and can also be readily protected from fibers and other solids in the stock, a simple means of controlled movement is provided, whereby more even pressure can be maintained across the width of the forming fabrics, thus reducing defects in the incipient paper web.
Thus in its broadest embodiment, this invention seeks to provide a flexible mounting for use in the forming section of a twin forming fabric paper making machine consisting essentially of:
a base member constructed and arranged to be supported by a paper making machine structure;
a flexible C-shaped beam, having a first beam portion having a first edge and having a second beam portion having a second edge, the first edge of the first beam portion being attached to the base member;
a fabric contacting blade attachment means, having a leading and a trailing face, attached to the second edge of the C-shaped beam; and
a pressurized loading tube located within the C-shaped beam between the base member and the second edge of the C-shaped beam.
Preferably, the first and second beam portions are both arcuate to provide the C-shaped beam. In a first alternative structure, the first beam portion has a second edge, the second beam portion has a first edge, the first and second beam portions are both substantially flat, and the second edge of the first beam portion is attached at an angle at its second edge to the first edge of the second beam portion to provide the C-shaped beam. In a second alternative structure, the first beam portion has a second edge, the second beam portion has a first edge, the first and second beam portions are both substantially flat, the second edge of the first beam portion is attached at an angle at its second edge to the first edge of the second beam portion to provide the C-shaped beam, and the first and second beam portions are a unitary structure.
Preferably, the base member and the C-shaped beam are fabricated as two separate units and attached together. Alternatively, the base member and the C-shaped beam are fabricated as a single unitary construction.
Preferably, when a two part construction is used, the first edge of the C-shaped beam is attached to the base member by being lodged into a slot in the base member constructed and arranged to receive the first edge, and the first edge is retained therein by a suitable adhesive. Alternatively, the first edge of the C-shaped beam is mechanically engaged to the base member by means of a slot in the base member constructed and arranged to receive the first edge, or by means of a direct mechanical attachment means.
Preferably, the mounting further includes a flexible sealing means located between the second edge of the C-shaped member and the base member. More preferably, the sealing means comprises a flexible strip of suitable width. Preferably, the edges of the sealing strip are engaged in cooperating slots in the second edge of the C-shaped beam and the base member.
Preferably, the pressurized loading tube is located in a slot constructed and arranged in the base member to receive it. More preferably, the pressure tube further includes a pressure rib adjacent the second edge of the C-shaped beam.
Preferably, the fabric contacting blade attachment means attached to the second edge of the C-shaped beam further includes a leading face and a trailing face, together with liquid venting means connecting the leading face to the trailing face. More preferably, the liquid venting means comprises a series of holes or slots from the leading face to the trailing face through the fabric contacting blade attachment means, the holes or slots being spaced apart in the cross-machine direction.